A liquid crystal display device (LCD), which is typically driven in a TN (twisted nematic), STN (super twisted nematic) or TFT (thin film transistor) mode, has been widely used in various electronic appliances including electronic watches, calculators, LCD TVs, and notebook computers, as well as in speed indication boards and movement systems of automobiles and airplanes. The LCDs are known to be advantageous in terms of a low electric power consumption, low driving voltage, light weight and thickness, while they are hampered by disadvantages such as narrow view angle, high operation temperature, and small display face size.
An LCD typically comprises a backlight unit, which functions as a light controlling apparatus for delivering light uniformly onto the entire face of LCD window. Accordingly, the backlight unit generally requires the emittance of a brighter light with minimized electric power consumption and a uniformity of brightness to the display window.
As shown in FIG. 1, a conventional backlight unit comprises lamp (1) for use as a light source, reflective plate (2) for reflecting light exited from the lamp, optical waveguide plate (3) for delivering light, a molded frame (not shown) for integrating the constitutional elements, light diffusive sheet (4) for enhancing the luminance and the view angle, and front panel (5).
The light diffusive sheet is typically prepared by coating a binder resin containing beads on the surface of a substrate to form a bead-containing light diffusive sheet, as disclosed, e.g., in Japanese Laid-open Patent Publication No. 1995-174902. In this method, the substrate is made of a polyester (e.g., polyethylene terephthalate), or poly-carbonate resin; the beads, an acrylic resin or glass; and, the binder resin, an acrylic or urethane-based resin. However, this method suffers from the disadvantage that the beads tend to drop out the sheet in the course of a sheet cutting process, which causes defects on the display screen comprising the sheet.
Further, there has been reported another method of preparing a light diffusive sheet, which involves forming a specified embossing on the surface of a substrate made of a thermoplastic resin. However, since this method requires the use of a roller mold having several tens micron sized embossing on the surface thereof, the production cost is high and it is difficult to fabricate an embossed mold which can attain the desired diffusion effect.
Further, Japanese Laid-open Patent Publication No. 2000-193805 discloses a process for preparing a light diffusive sheet comprising laminating a UV-curable resin film on the surface of a substrate using a mold having a surface with a plurality of parts each curved like a lens, pressing the resin on the surface of the mold to transcribe the lens pattern from the mold to the resin, curing the resin by UV irradiation, and releasing the mold from the resin. This method has the problems that bubbles may be incorporated in the resin layer upon laminating the UV-curable resin film, the patterns of the sheet obtained are easily damaged due to the use of UV curable resin in the form of a film, and the luminance may unsatisfactorily low because the light diffusive layer replicated from the mold has a reversed lens figure.